1. Field of the Invention
The present invention generally relates to systems and methods for determining a height of a specimen. Certain embodiments relate to systems and methods that may include a dual beam symmetric height sensor coupled to a processing tool, a metrology tool, or an inspection tool.
2. Description of the Relevant Art
Fabricating semiconductor devices such as logic and memory devices may typically include processing a specimen such as a semiconductor wafer using a number of semiconductor fabrication processes to form various features and multiple levels of the semiconductor devices. For example, lithography is a semiconductor fabrication process that typically involves transferring a pattern to a resist arranged on a semiconductor wafer. Additional examples of semiconductor fabrication processes may include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated in an arrangement on a semiconductor wafer and then separated into individual semiconductor devices.
As feature sizes of semiconductor devices continue to shrink, minimum feature sizes that may be successfully fabricated may often be limited by performance characteristics of semiconductor fabrication processes such as lithography and etch processes. Examples of performance characteristics of a lithography process include, but are not limited to, resolution capability, across chip linewidth variations, and across wafer linewidth variations. In optical lithography, such performance characteristics may be determined by a number of process parameters such as the quality of resist application, the performance of the resist, the exposure tool, and the wavelength of light which is used to expose the resist. The ability to resolve a minimum feature size, however, may also be strongly dependent on other critical parameters of lithography processes such as a temperature of a post exposure bake process or an exposure dose of an exposure process. As such, controlling the parameters of processes which are critical to the resolution capability of semiconductor fabrication processes such as lithography processes is becoming increasingly important to the successful fabrication of semiconductor devices.
Controlling critical process parameters may typically include assessing the performance characteristics of semiconductor fabrication processes such as resolution capability, across chip linewidth variations, and across wafer linewidth variations. As the dimensions of semiconductor devices continue to shrink with advances in semiconductor materials and processes, however, the ability to examine microscopic features and to detect microscopic defects in semiconductor devices has become increasingly difficult. Significant research has been focused on increasing resolution limits of metrology tools that are used to examine microscopic features and defects. Optical microscopes generally may have an inherent resolution limit of approximately 200 nm and may have limited usefulness in current manufacturing processes. Microscopes that utilize electron beams to examine devices, however, may be used to investigate feature sizes as small as, e.g., a few nanometers. Therefore, tools that utilize electron beams to inspect semiconductor devices are becoming integral to semiconductor fabrication processes. For example, in recent years, scanning electron microscopy has become increasingly popular for the inspection of semiconductor devices. Scanning electron microscopy typically involves scanning an electron beam over a specimen and generating an image of the specimen by detecting electrons that are reflected, scattered, or transmitted from the specimen.
During each semiconductor device fabrication process, defects such as particulate contamination and pattern defects may be introduced into the semiconductor devices. Such defects may be isolated to a single semiconductor device on a semiconductor wafer containing several hundred semiconductor devices. For example, isolated defects may be caused by random events such as an unexpected increase in particulate contamination in a manufacturing environment or an unexpected increase in contamination in process chemicals that may be used in fabrication of the semiconductor devices. Alternatively, the defects may be repeated in each semiconductor device formed across an entire semiconductor wafer. In an example, repeated defects may be systematically caused by contamination or defects on a reticle. A reticle, or a mask, may be disposed above a semiconductor wafer and may have substantially transparent regions and substantially opaque regions that are arranged in a pattern that may be transferred to a resist on the semiconductor wafer. Therefore, contamination or defects on a reticle may also be reproduced in the pattern transferred to the resist and may undesirably affect the features of each semiconductor device formed across an entire semiconductor wafer in subsequent processing.
Defects on semiconductor wafers may typically be monitored manually by visual inspection, particularly in the lithography process because many defects generated during a lithography process may be visible to the naked eye. Such defects may include macro defects that may be caused by faulty processes during this step. Defects that may be visible to the human eye typically have a lateral dimension greater than or equal to approximately 100 xcexcm. Defects having a lateral dimension as small as approximately 10 xcexcm, however, may also be visible on unpatterned regions of a semiconductor wafer. An example of a visual inspection method is illustrated in U.S. Pat. No. 5,096,291 to Scott and is incorporated by reference as if fully set forth herein. Prior to the commercial availability of automated defect inspection systems such as the systems illustrated in U.S. Pat. No. 5,917,588 to Addiego and U.S Pat. No. 6,020,957 to Rosengaus et al., which are incorporated by reference as if fully set forth herein, manual inspection was the most common, and may still be the most dominant, inspection method used by lithography engineers.
Automated inspection systems were developed to decrease the time required to inspect a wafer surface. Such inspection systems may typically include two major components such as an illumination system and a collection-detection system. An illumination system may include a light source such as a laser that may produce a beam of light and an apparatus for focusing and scanning the beam of light. Defects present on the surface may scatter the incident light. A detection system may detect the scattered light and may convert the detected light into electrical signals that may be measured, counted, and displayed on an oscilloscope or other monitor. Examples of such inspection systems are illustrated in U.S. Pat. No. 4,391,524 to Steigmeier et al., U.S. Pat. No. 4,441,124 to Heebner et al., U.S. Pat. No. 4,614,427 to Koizumi et al., U.S. Pat. No. 4,889,998 to Hayano et al., and U.S. Pat. No. 5,317,380 to Allemand, all of which are incorporated by reference as if fully set forth herein.
Systems used to manufacture semiconductor devices such as processing tools, metrology tools, and inspection tools may include a height sensor. A height sensor may be used to position a wafer within a system prior to the processing of the wafer. Height sensors may be used in different configurations for different applications. For example, height sensors may be used in wafer probe applications. Examples of such height sensors are illustrated in U.S. Pat. Nos. 4,328,553 to Fredriksen et al., and 5,948,972 to Samsavar et al., which are incorporated by reference as if fully set forth herein. In addition, height sensors may be used in wafer inspection applications. Examples of such height sensors are illustrated in U.S. Pat. Nos. 6,107,637 to Watanabe et al., 6,140,644 to Kawanami et al., and 6,172,365 to Hiroi et al., all of which are incorporated by reference as if fully set forth herein.
Further examples of height sensors configured for use in metrology tools, microscopes and lithography tools may be found in U.S. Pat. Nos. 4,788,431 to Eckes et al., 5,585,629 to Doran et al., and 6,208,407 to Loopstra, which are incorporated by reference as if fully set forth herein. For example, Loopstra discloses that in measurement of a height of each substrate field, the field and a first height sensor are moved with respect to each other in a plane perpendicular to the axis of a projection beam. A second height sensor is used for measuring a height of a substrate support reference plane and a height of the substrate support reference plane associated with an ideal height of a relevant substrate field is subsequently computed and stored. After the substrate has been introduced into the projection beam, the value of the height for each substrate field is checked by means of a third height sensor.
Several height sensors that may be used as focusing sub-systems for processing, metrology, and inspection systems are currently available. Many of these height sensors may be coupled on-axis to a system in a through-the-lens configuration. The term, xe2x80x9cthrough-the-lens configuration,xe2x80x9d as used herein, generally refers to a system in which the height measurement location is substantially the same as a sample location of a lithographic or metrology system, and in which light of the height sensor passes through at least one lens of the lithographic or metrology system. Therefore, many of these height sensors may be of limited use to systems with mechanical and/or physical constraints. Other height sensors are currently available that may be coupled on- or off-axis to a system. The term, xe2x80x9con-axis height sensor subsystem,xe2x80x9d as used herein, generally refers to a system in which the height measurement location is substantially the same as a sample location of a lithographic or metrology system. The term xe2x80x9coff-axis height sensor sub-system,xe2x80x9d as used herein, generally refers to a system in which the height measurement location is not the same as a sample location of a lithographic or metrology system. Such height sensors, however, may not be able to achieve a high level of precision due to wafer pattern-induced measurement errors.
Patterns on a specimen such as topographical features, which may be formed upon or within a specimen such as a wafer, diffraction effects, and/or thin film interference effects may affect a height sensitive image such as a spot projected onto a detector. Such a detector may include, for example, a position sensitive detector (xe2x80x9cPSDxe2x80x9d), or device. A location of a centroid on a PSD may be used to determine the height of the specimen. Patterns of the specimen may reduce an intensity of the image on the PSD by up to about 90%. If the patterns also cause intensity variations across the image on the PSD, an apparent location of the centroid may be shifted. Therefore, such a shift in the apparent location of the centroid may introduce error into height determinations of the specimen. In a worst case scenario, for example, an apparent location of the centroid may be shifted laterally by as much as one-fourth of the spot size of the light on the surface of the specimen, as measured perpendicular to an axis of the height sensor sub-system.
Increasing demands for smaller device geometries, higher throughput and yield, and lower manufacturing costs in semiconductor device manufacturing typically leads to increased precision and speed requirements for height sensor sub-systems of processing, metrology, and inspection systems. An embodiment of a height sensor sub-system, as described herein, may include a dual beam symmetric height sensor configured to assess, or determine, a height of a specimen such as a semiconductor wafer. The height sensor may generate an error signal that may be used to drive a Z-axis fine height adjustment of a system such as a processing, metrology, or inspection system. In this manner, a substantially constant working distance may be maintained between a surface of the specimen and an optical column of the system. The symmetrical optical design of the system may reduce, or even substantially eliminate, pattern-induced error in the determination of height. For example, the optical design of the system may be configured such that a height of a specimen or wafer having a substantially un-symmetric surface may be measured with relatively high sensitivity. Therefore, such a height sensor may be used to position a specimen within a system with high precision (i.e., on the order of less than about +/xe2x88x921 xcexcm) by eliminating pattern-induced error in the error signal.
Systems, as described herein, may be configured to maintain an absolute height of a specimen in approximately real time. For example, the height sensor may be used during inspection of a wafer to maintain a substantially focused beam of light on the wafer while a stage on which the wafer is located may be moving. In this manner, generating a focus map of the specimen prior to inspection may be eliminated. For example, generating a focus map may include assessing a height of a specimen at a plurality of locations across the specimen prior to processing, metrology, or inspection. The generated focus map may then be used during processing, metrology, or inspection to alter a height of the specimen depending on the location on the specimen being processed, measured, or inspected. Generating such a focus map increases process time. Therefore, a height sensor sub-system, as described herein, may reduce process time thereby increasing throughput of such systems.
Such a height sensor may be coupled in an on- or off-axis configuration to a system. In this manner, the height sensor may comply with mechanical and physical constraints of a process, metrology, or inspection tool.
In an embodiment, a system may be configured to assess a height of a specimen. The system may include an illumination system configured to direct a first and a second beam of light to a surface of the specimen at substantially opposite azimuth angles and at substantially equal angles of incidence. In this manner, the first and second beams of light may illuminate substantially the same area of the surface of the specimen. The illumination system may include a light source configured to emit broadband light such as a metal halide lamp. Alternatively, the illumination system may include an optical fiber coupled to a light source. In addition, the illumination system may also include two apertures illuminated by a light source. Each of the two apertures may include an opening having a diameter of approximately 400 xcexcm to approximately 800 xcexcm. For example, each of the two apertures may include an opening having a diameter of approximately 600 xcexcm.
The illumination system may be configured to direct the first and second beam of light at a relatively shallow angle of incidence with respect to a surface of the specimen. For example, an angle of incidence between the first and second beams of light and a surface of a specimen may be about 4xc2x0 to about 10xc2x0. The angles of incidence, however, may range from about 1xc2x0 to about 45xc2x0 depending on physical limitations of the system. Additionally, the illuminated area of the surface of the specimen may include an elliptical-shaped spot, a rectangular-shaped spot, or a square-shaped spot. A shape of the spot may vary depending on, for example, the illumination system and/or the apertures.
In a further embodiment, the system may include a collection system configured to collect the first beam of light specularly reflected from the surface of the specimen and to collect the second beam of light specularly reflected from the surface of the specimen. For example, the collection system may include two imaging lenses. Specularly reflected portions of the first and second beams of light may propagate along substantially opposite azimuth angles and along substantially equal angles of incidence. Therefore, the two imaging lenses may be positioned at substantially opposing azimuth angles and at substantially equal angles of incidence.
In an embodiment, the system may also include a first and a second beam splitter. The first beam splitter may be configured to direct the first collected beam of light to a first detector. The second beam splitter may be configured to direct the second collected beam of light to a second detector. The first and second beam splitters may also be replaced with half-silvered mirrors. The first and second detectors may be PSDs, CCD arrays, or TDI cameras. The first and second detectors may also include any detector configurable to determine a position of a light beam on the detector. The detectors may be configured to generate output signals. For example, the first detector may be configured to generate an output signal responsive to the first collected beam of light. In addition, the second detector may be configured to generate an output signal responsive to the second collected beam of light. Furthermore, the output signal generated by the first detector may be responsive to a position of the collected first beam of light on the first detector. Similarly, the output signal generated by the second detector may be responsive to a position of the collected second beam of light on the second detector.
In an additional embodiment, the system may include a device such as a differential amplifier. The device may be configured to receive the output signals from the first and second detectors. In addition, the device may be configured to generate a comparison, or error, signal from the output signals. The comparison signal may be responsive to the height of the specimen. The comparison signal may also be substantially independent of patterned features on the specimen. In this manner, the comparison signal may have a height precision of less than about +/xe2x88x921 xcexcm.
In an embodiment, the system may include a processing device. The processing device may be configured to receive the comparison signal. In addition, the processing device may be configured to alter the height of the specimen in response to the comparison signal. For example, the processing device may be configured to alter the height of the specimen by altering a position of a stage supporting the specimen or by adjusting a focus of the optical column to bring the specimen into focus. The processing device may be, for example, an inspection, metrology, or process tool. The tool may include a closed-loop system to maintain a substantially constant working distance between an optical column and the specimen during inspection, metrology, or processing.
Additional embodiments relate to a method for assessing, or determining, the height of a specimen. The method may include directing a first and a second beam of light to a surface of the specimen at substantially opposite azimuth angles and at substantially equal angles of incidence. In this manner, the first and second beams of light may illuminate substantially the same area of the surface of the specimen. The first and second beam may be directed to the surface at an angle of incidence of approximately 4xc2x0 to approximately 10xc2x0. The angles of incidence, however, may range from about 1xc2x0 to about 45xc2x0 depending on physical limitations of the system. The illuminated area of the surface of the specimen may be an elliptical-shaped spot, a rectangular-shaped spot, or a square-shaped spot depending on, for example, a configuration of the illumination system. The illumination system may be configured as any of the embodiments described herein.
In a further embodiment, the method may include collecting the first and second beams of light specularly reflected from the surface of the specimen. The method may also include directing the collected first beam of light to a first detector and directing the second collected beam of light to a second detector. The first and second detectors may be configured as described herein. In addition, the method may include generating an output signal responsive to the first collected beam of light and generating an output signal responsive to the second collected beam of light. Furthermore, generating an output signal may be further responsive to a position of the first collected beam on the first detector and further responsive to a position of the second collected beam on the second detector.
The method may further include generating a comparison signal from the output signals. For example, the method may include sending the output signals responsive to the first and second collected beams of light to a differential amplifier. The comparison signal generated by the differential amplifier may be responsive to the height of the specimen. In addition, the comparison signal may be substantially independent of patterned features on the specimen. In this manner, the comparison signal may have a height precision of less than about +/xe2x88x921 xcexcm.
In an embodiment, the method may include altering the height of the specimen in response to the comparison signal. For example, altering the height of the specimen may include maintaining a substantially constant working distance between an optical column of a system and the specimen during processing, metrology, or inspection. In addition, altering the height of the specimen may include altering a position of a stage configured to support the specimen. Alternatively, the method may include altering a focus setting of an optical column in response to the comparison signal.
An additional embodiment relates to a semiconductor device that may be fabricated by a method. The method for fabricating the semiconductor device may include forming a portion of the semiconductor device upon a specimen such as a wafer. In addition, the method may also include any of the embodiments described above.